1. Field of the Invention
The invention relates to techniques for the manufacture of an oxidation resistant, high strength titanium alloy which may be in the form of a flat rolled or coiled strip product. The techniques are advantageously used for the manufacture of an alloy product ideal for use in automotive exhaust systems components, wherein elevated temperature strength and oxidation resistance are a required combination of properties.
2. Background of the Invention
It is known to use commercially pure (CP) titanium for automotive exhaust systems and mufflers for motorcycles. These exhaust systems made of CP titanium are lighter than those made from standard stainless steel. Weight reductions when using titanium to replace stainless steel may be as high as 44%, which can be equivalent to or larger than approximately 20 lbs. of weight reduction for the system.
The use of CP titanium in exhaust systems, while providing the benefit of good weight reduction, on the other hand results in the CP titanium exhibiting excessive oxidation and softening due to the high temperatures associated with this application. Consequently, the use of CP titanium sheet product has been limited to specific components of exhaust systems that are exposed to relatively low temperatures.
Where exhaust pipes are made from titanium they generally include a welded tube manufactured from CP titanium. In the case of muffler and catalytic converter boxes, the components can be manufactured from sheets of CP titanium by forming and welding. The input material for tube and muffler components has typically been produced as a continuous cold rolled strip product. The known process to produce a titanium strip product includes melting an ingot, converting the ingot to an intermediate slab by hot forging or rolling, then rolling the slab from a high temperature to coil sheet product or hot band coil through a series of reducing roll gaps. This can be accomplished through a sequence of rolling mills assembled in tandem or in a reversing mill, as is well known in the art.
The hot band coil is also typically heat treated or annealed in a continuous line furnace and further can be trimmed and treated to remove surface contamination and cracks. The hot band coil is then cold rolled to final gage on a coil rolling mill such as a Sendzimir mill. After rolling the coil can be annealed in a continuous inert gas or vacuum line furnace or in a bell furnace under vacuum or inert gas and finally the cold rolled coil or strip is finished for sale with additional steps that can include leveling, and acid pickling.
In the manufacture of welded tubes for the pipe components of an exhaust system, the cold rolled strip can be slit into appropriate widths and either fed into a continuous tube welding line with roll formers and an autogenous welding source such as tungsten inert gas (TIG), metal inert gas (MIG) or laser welding, or cut to length formed to tube and welded as individual lengths. For these processes, the preferred characteristics for the strip product are a smooth low friction surface to prevent the forming tools from sticking on the strip, a smooth yield curve in the transverse direction to facilitate uniform forming into the tube shape and sufficient bend ductility to form the tube. The welded tube should also have sufficient formability to be bent into the final desired exhaust pipe shapes and have sufficient mechanical (e.g., strength) and oxidation performance characteristics to withstand exposure to the exhaust gas for the intended life of the pipe components.
For the manufacture of muffler components and catalytic converter boxes, the coil or strip will typically be cut into flat sheets from which individual blanks can be cut before forming and assembly which can involve combinations of deep drawing, pressing, bending, forming and rolling lock seams and welding as necessary. For the manufacture of the muffler components, the key characteristics are formability in drawing and pressing, and excellent bend ductility. The selected material should have sufficient mechanical (e.g., strength) and oxidation performance characteristics to withstand exposure to the exhaust gas for the intended life of the muffler components.
The combination of performance characteristics required for the above-mentioned products is not straight forward. The ideal selection of titanium alloy from a manufacturing standpoint would be a soft commercially pure grade of titanium such as ASTM grade 1 or ASTM grade 2. However, such alloys have limited oxidation life and insufficient high temperature mechanical performance for the current vehicles. Moreover, the next generation of fuel efficient engines is likely to develop even higher temperatures and loads.
Techniques for the production of alloys with improved mechanical and oxidation performance are thus required to meet the needs of the industry for a titanium alloy that can be used at higher temperatures than CP titanium sheet product. The important properties for this product are oxidation resistance and elevated temperature strength at temperatures up to 1600° F. In addition, since this sheet product requires a forming and fabricating operation to produce the various exhaust system components, cold formability and weldability are required to be near the properties exhibited by CP titanium.